Radio frequency (RF) power amplifiers, such as class AB, bipolar junction transistor (BJT) amplifiers, are known to introduce amplitude and phase distortion in an amplified output signal due to the inherent nonlinear characteristics of BJT devices. Such distortion typically includes high order nonlinearities (e.g., fifth order, seventh order, and ninth order intermodulation products) when the BJT device is driven with multiple tone, high voltage input signals, as is the case for base station amplifiers at a typical cellular base site. When transmitted, the distortion creates unwanted interference in the transmission bandwidth of the cellular system--for example, at the frequencies corresponding to the fifth order, seventh order, and ninth order intermodulation products.
To reduce the nonlinear distortion produced by high power RF amplifiers, amplifier designers generally use a linearization technique, such as a feed forward technique. In one feed forward realization, an input signal to an RF amplifier is sampled prior to amplification by the RF amplifier. An RF pilot reference signal is combined with the input signal subsequent to the sampling of the input signal. The input signal and the RF pilot signal are applied to the RF amplifier. The RF amplifier amplifies both signals and introduces nonlinear amplitude and phase distortion into both signals during the amplification process. The amplified signal is sampled and the sampled input signal is subtracted from the sampled amplified signal to extract the distortion in the sampled amplified signal. This extracted distortion is commonly called an error signal.
The error signal is adjusted in amplitude and phase based on the level of the RF pilot signal detected with an RF pilot receiver at the output of the feed forward amplifier network. The adjusted error signal is amplified and subtracted from the originally amplified signal to produce a corrected signal having less distortion than the originally amplified signal. The corrected signal serves as the output signal of the feed forward network. Thus, the feed forward approach reduces the distortion introduced by the high power RF amplifier circuit as indicated by a reduction in the level of the RF pilot signal in the output signal.
The RF pilot signal is used to monitor the level of nonlinear distortion contained in the corrected signal. However, complex and costly RF circuitry (e.g., an RF oscillator, an RF amplifier, and an RF receiver) is required to generate and receive the RF pilot signal. Therefore, a need exists for a method and apparatus for reducing distortion in an output signal of an amplifier circuit that does not use an RF pilot signal as a reference for monitoring the distortion in a feed forward configuration.